These types of valves are employed, for instance, as dynamic lowering brake/block valves for regulating the outflow of a hydraulic device that is loaded by external forces (a lifting cylinder, for example) in order to prevent the hydraulic device from getting ahead of the feed flow. At the same time, in cases in which they are designed as seat pistons, these valves assume the function of a leakage-free, blocking, steered non-return valve that clamps the hydraulic device in position.
It is known from the prior art pursuant to DE-OS 32 39 930 which is a brake/block valve that in a special exemplary embodiment is designed with a positive opening non-return valve. This known brake/block valve comprises a first hollow valve piston having an interior valve seat that can be closed via a valve plunger that is pre-loaded in the closed position. The interior valve seat is in communication with a first pump-side valve pressure connection via a first radial bore provided in the valve piston. A second radial bore arranged behind the valve seat is connected via an annular channel to an additional valve seat of the non-return valve, which blocks a second hydraulic device-side valve pressure connection. A pressure limiting valve is also integrated into the non-return valve. The aforesaid valve piston furthermore has additional small diameter radial bores immediately in front of the interior valve seat and is pre-loaded via a spring in a position in which communication between the annular channel and the small diameter radial bores is interrupted.
For lifting a load, hydraulic fluid flows from the first valve pressure connection through the valve piston, the interior valve seat, the annular channel, the valve seat of the non-return valve to the second valve pressure connection and is forwarded from there to the hydraulic device. If the load is to be maintained in a certain position, the non-return valve is pressed onto the valve seat via a spring; when the pumping pressure decreases, the hydraulic pressure acts on the second valve pressure connection and therefore on the hydraulic device. For lowering the load in this known device, the valve piston and the piston of the non-return valve are caused to move against the pre-loaded spring by the pressure in a control pressure line, a fluid connection being produced between the second valve pressure connection (via the valve seat of the non-return valve, now positively opened, the annular channel, and the small diameter radial bores) and the first valve pressure connection, which in this case is connected to a tank via an external directional control valve.
From the preceding description of the prior art it can be seen that the overall structure of the brake/block valve is extremely complex given the two valve pistons, each of which must be actuated via a control pressure, and this fact leads to relatively large structural dimensions and furthermore to increased pressure losses caused by leakage.
A brake/block valve that is the equivalent of that illustrated in DE-OS 2 352 742 was developed with the goal of keeping losses due to leakage as small as possible; it is the prior art closest to the subject of the invention.
This valve comprises a hollow valve piston that is pre-loaded via a spring against a valve seat that connects two valve pressure connections, the spring being located in a spring area. Also located in the valve piston is an internal pilot seat valve that can be positively opened by means of a setting piston that can be actuated by a control pressure. The brake/block valve is also parallel-switched with both of its connections to a non-return valve arranged in a pressure line leading to the hydraulic device, the spring area being permanently subjected to the load pressure.
When a load is lifted, the pressure connection located upstream of the non-return valve is subjected to pump pressure that is elevated relative to the load pressure prevailing in the connection located downstream of the non-return valve, corresponding to the force of the spring in the non-return valve, the valve piston being pressed by both the piston spring and the load pressure prevailing in the spring area on the valve seat. If the load is to be maintained in one position, the external non-return valve closes, while the valve piston of the brake/block valve is further held in the closed position and therefore the pump-side valve connection is essentially without pressure. For lowering the load, the setting piston is actuated for opening the internal pilot seat valve, this releasing the pressure in the spring area. Additional actuation of the setting piston causes the valve piston to be raised by the valve seat and therefore the hydraulic device pressure is released into a tank.
It has been demonstrated that in this type of valve design the varying level of the pressure on the pump-side valve connection has a negative effect on the control of the valve piston. In addition, the brake/block valve housing (especially for the spring area) must be heavy and particularly well-sealed against fluids since it is subjected to the load pressure continuously and therefore must be able to withstand it.
Given this most recent prior art, it is therefore the object of the invention to create a brake/block valve, preferably as a pipe rupture valve, the structure of which is less complex and less expensive.